The present invention relates to a piezoelectric pressure sensor and more particularly but not exclusively relates to a piezoelectric pressure sensor suitable for detecting combustion pressure within a cylinder of the internal combustion engine, for example.
The piezoelectric pressure sensor utilizing piezoelectric effect in which electric charges are generated by applying stress has been widely used. Recently, a piezoelectric pressure sensor suitable for detecting combustion pressure within a cylinder of the internal combustion engine has been intensively developed.
FIG. 3 is an axial section of a piezoelectric pressure sensor disclosed in Japanese Patent Unexamined Publication No. 63-109342. The piezoelectric pressure sensor is of the type in which a shearing stress is applied to a piezoelectric element 24 thereof. The piezoelectric element 24 is arranged within a sensor housing 21. The piezoelectric element 24 is secured and prestressed in such a manner that the piezoelectric element 24 is urged by a fastening screw member 25 against a pressure receiving surface 22 through a pressure transmitting member 23. The pressure is necessary for measuring negative pressure particularly in measuring combustion pressure within a cylinder of the internal combustion engine. The pressure applied to the pressure receiving surface 22 is transmitted to an inner circumferential portion of the piezoelectric element 24 through the pressure transmitting member 23 which is electrically insulated from the sensor housing 21. In this event, a shearing stress is produced in the piezoelectric element 24 since the piezoelectric element 24 is held at an upper outer circumferential portion thereof by the upper fastening screw 25. Thus, electric charges are generated in the inner and outer circumferential surfaces of the piezoelectric element 24 according to the shearing stress, and the electric charges are detected as an electric signal from electrodes provided on the inner and outer circumferential surfaces. The piezoelectric pressure sensor with such a construction is advantageous in that it has a simple structure. However, when the piezoelectric pressure sensor is threaded to a test article 32, tensile stresses are produced in the flange of the sensor housing 21 in proportion to a tightening force of the sensor housing 21 to the test article 32 with tensile stresses generated as shown in FIG. 4, so that the distance between the pressure receiving surface 22 and the upper fastening screw 25 is enlarged. As a result, the prestress applied to secure or fix the piezoelectric element 24 is rather reduced, and hence contacts between the pressure transmitting member 23 and the piezoelectric element 24 and between the piezoelectric element 24 and the fastening screw 25 become unstable. This causes the pressure in the pressure receiving surface 22 to be nonuniformly transmitted or little transmitted to the piezoelectric element 24, resulting in a considerable fluctuation of the sensor output. Moreover, the sensor output is adversely affected since the piezoelectric element rather changes in piezoelectric constant according to the magnitude of the prestress.